The present disclosure relates to a write-once optical recording medium. Specifically, the present invention relates to a write-once optical recording medium having an inorganic recording layer and a process for manufacturing the same.
CDs (Compact Discs), DVDs (Digital Versatile Discs), and the like have led the market of optical recording media. However, recently, as a realization of high-vision televisions and a rapid increase in the volume of data handled by PCs (personal computers), it has been desired that optical recording media have a larger capacity. In order to meet this requirement, optical recording media for a blue laser, such as BDs (Blu-ray discs (registered trademark)) and HD-DVDs (Heigh-Definition Digital Versatile Discs) have appeared, and a new market of optical recording media having a large capacity has been established.
Recordable optical recording media include rewritable optical recording media represented by a CD-RW (Compact Disc-ReWritable) and a DVD±RW (Digital Versatile Disc±ReWritable), and write-once optical recording media represented by a CD-R (Compact Disc-Recordable) and a DVD-R (Digital Versatile Disc-Recordable). In particular, the latter optical recording media have made a large contribution to market expansion as low-price media. Accordingly, also in large-capacity optical recording media for a blue laser, in order to expand the market, it is believed that a reduction in the price of write-once optical recording media is necessary.
In write-once optical recording media, both inorganic materials and organic dye materials are permitted as recording materials in terms of the standard. Write-once optical recording media including an organic material are advantageous in that they can be produced by a spin-coating method at a low cost. On the other hand, write-once optical recording media including an inorganic material are advantageous in that they have good reproduction durability and push-pull signal characteristics but are disadvantageous in that a large sputtering apparatus is necessary. Accordingly, in order that write-once optical recording media including an inorganic material can be competitive with those including an organic material in terms of price, it is essential that an initial investment for a production apparatus be reduced and the takt per disc be increased to efficiently produce the recording media.
An example of the most effective means for solving the above problem is reducing the number of layers constituting a recording film to reduce the number of film deposition chambers, thus reducing the initial investment for a sputtering apparatus and reducing a production takt. However, even if the number of layers is merely reduced, when the film thickness is large and a material whose film deposition rate is low is used, the production takt increases, which may actually result in an increase in the cost.
Hitherto, as materials of protective layers of write-once optical recording media including an inorganic material, transparent dielectric materials such as SiN and ZnS—SiO2 have been mainly used (refer to, for example, Japanese Unexamined Patent Application Publication No. 2003-59106). Although SiN and ZnS—SiO2 are advantageous in that the film deposition rate is high and thus productivity is good, they have a problem that a storage characteristic (storage reliability) of recorded data is poor. Among dielectric materials other than those mentioned above, there are dielectric materials that have high storage reliability. However, it is necessary to deposit these materials by radio-frequency (RF) sputtering, and thus there is a problem that the film deposition rate is very low and productivity is low. Accordingly, it is very difficult to satisfy both storage reliability and productivity.